Odd problem with D-400 engine Lawnboy Model 5083

[Phototone]

I just rebuilt this Model 5083 Lawnboy with a D-400 series engine. I put new rings in it, among other things. This has a 2-ring piston. I started it and adjusted it and mowed some with it, and after I finished one small yard, I noticed when I switched it off, that it appeared to be dragging (and no brake on this early model). I tried moving the blade and it seemed the piston was "almost" frozen, or locked up, but not quite. The engine was quite hot. Now I used STP as assembly lube when I reassembled the piston into the cylinder. I took out the spark plug and squirted some light oil into the cylinder and worked the piston back and forth using my hand on the mower blade, and got it freed up. Now I was fueling the mower with my standard mix of 8 oz. Lawnboy oil, to 2 galllons non-ethanol gas, with a dollop of Marvel Mystery Oil. It was my understanding that the 2-ring piston Lawnboy used this ratio of oil which is 1:32. This engine has a roller bearing top bearing and a sleeve lower bearing. Anyway, I mixed up a fresh gallon of fuel with double the lawnboy oil 1:16, and continued mowing and it seems to be OK. I find this curious and can't explain it, unless this mower "actually" needs the 1:16 ratio. Any ideas why it almost froze up? In any case I'm continuing with the 1:16 oil ratio for now.

 

[Rivets]

If my memory is still working Lawn Boy didn't switch to a 32/1 fuel mix until the mid seventies when the Series F engines came out. The series C and D engines used a 16/1 mix. There are a couple of Lawn Boy lovers on this forum who will be able to tell us if I'm right or wrong.

 

[Kerry]

Whether a mower, outboard motor, motorcycle, etc. ALL the wise old sages say that when reringing or rebuilding a two stroke motor, run it with double oil to seat the rings and wear in the bearings . Run a few tanks of double mix, then you should be good.

 

[jp1961]

Piston installed correctly? The word TOP cast into the piston skirt should face away from the deck.

Jeff

 

[Phototone]

Piston installed correctly? The word TOP cast into the piston skirt should face away from the deck.

Jeff

Yes, I didn't take the piston off the con-rod or crankshaft, so it could only go back the way it was intended. I just removed the cylinder, exposing the piston.

UPDATE: So far, running on 1:16 oil ratio, its running fine, and no smoking.

 

[motoman]

Piston bore clearance is a tricky thing. For instance going from cast to forged or a new mfg can cause problems. Bore clearance difference between measured bore ID and piston accross the skirts? Some require a feeler gauge and a "fish deliar" pull . Just ideas.

 

[Phototone]

Piston bore clearance is a tricky thing. For instance going from cast to forged or a new mfg can cause problems. Bore clearance difference between measured bore ID and piston accross the skirts? Some require a feeler gauge and a "fish deliar" pull . Just ideas.

Please note: I did not change the piston or cylinder. I merely separated them, and installed new rings on the existing piston, then put back together. I did not take the piston off the con-rod, nor did I remove the con-rod from the crankshaft. Minimal changes here.....just new rings. The cylinder was not scored or otherwise damages, likewise, the piston was fine. The mower was working before, it just had low compression, which the rings cured. I also did other external things, like new points, new seals, rebuilt carb, new blade, etc.

 

[motoman]

photo, Good show and hope it evens out for you.

 

[LB8210]

Lawn Boy service manual shows a 32to1 mix for 1972 and up motors and 16to1 for 1971 and back motors. In 1970 they changed to a 2 ring piston with needle bearings at both ends of the rod. 1969 and older motors used a three ring piston with a bushing at the piston end of the rod. When they added needles to the piston end of the rod AND changed the oil quality the motor was then a 32to1 mix motor. The combination of these two changes in 1972 made using a 32to1 mix ok. Using the current LB quality oil in a 1970 or 1971 motor would be ok at a 32to1 ratio because they have the needles at both ends. Now back to the motor running hot and getting tight. When you installed new rings , what was the end gap of the new rings?

 

[beg]

so it would be safe to say you did not hone the cylinder or cross hatch the piston skirt?